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00:00There's a killer lurking in our galaxy, a star ready to explode into a supernova.
00:12These are the most visually stunning events in the universe.
00:15Seen from Earth, it would have a terrible beauty, but for us, it could be fatal.
00:21In a few seconds, it can release as much energy as the sun will over its entire lifetime.
00:28We're trying to hunt it down, but it's lying low.
00:31We haven't seen a supernova in the Milky Way in over 400 years.
00:36It could be anywhere.
00:41It is nearly impossible to predict where and when the next supernova will happen.
00:48The hunt is on to find the next supernova, before it finds us.
01:12October 2019.
01:15One of the brightest stars in the sky looks dangerously unstable.
01:23If you look at the constellation of Orion, one of the shoulders of Orion is a star that
01:28is obviously red.
01:31This is Betelgeuse.
01:39I could go into my backyard and see it.
01:40You could clearly see that it was getting dimmer.
01:44Is this a warning?
01:47Is Betelgeuse about to die in a massive cosmic explosion?
01:52A supernova?
01:55We've been studying this star for hundreds of years, and one thing we're sure about is
02:00that it's big, very big.
02:06Betelgeuse is a massive star, maybe 15 or 20 times the mass of our sun, and it's near
02:11the end of its life.
02:14It is a massive, enormous, luminous star, and one day, it's going to go boom.
02:24Betelgeuse is on our list of supernova candidates because of this massive size.
02:32The bigger star they are, actually the shorter the lifespan.
02:38The lifespan of a star depends on a delicate balance between two competing forces, gravity
02:45pulling in and heat and pressure pushing out.
02:51Stars exist because they're held up.
02:54They're not held up by pillars.
02:56They're held up by energy flowing out of the core toward the surface of the star.
03:01That stops the gravitational contraction.
03:04Stars get their energy from nuclear fusion reactions right in the core, and the most
03:08basic one is taking two hydrogen atoms and slamming them together to form a helium atom.
03:13And you might think, okay, the more hydrogen you have, the more stuff you have, maybe the
03:17longer the star will live.
03:19It turns out it's exactly opposite.
03:22The reason?
03:23Gravity.
03:24The more mass a star has, the stronger its gravity.
03:29Gravity that crushes its hydrogen atoms closer together.
03:34As you crush things more and more, the temperature gets hotter and hotter and hotter, and the
03:38nuclear fusion reactions burn faster.
03:41So bigger stars burn their fuel very, very quickly and live short lives.
03:46Smaller stars burn their fuel much more slowly and live long, protracted lives.
03:50So when you are a big star, you live fast and you die young.
04:01Fuel juice burns brighter than 125,000 suns, but now it's running out of its hydrogen fuel.
04:10So it's burning whatever it has left just to stay alive.
04:17Stars are basically factories for burning hydrogen into helium.
04:21And then once the helium is burned, they start burning heavier and heavier elements like
04:27carbon and nitrogen and oxygen.
04:32It's a little like you burn something, you get ash, but then if you crush the ash enough,
04:36you can burn it again.
04:38And then you crush it some more and you can burn it yet again.
04:43But this process can't go on forever.
04:47As the size of the atomic nuclei being fused together grows, the amount of energy released
04:52falls.
04:54The fuel the star needs to resist the pull of gravity is running out.
05:00Unfortunately, the amount of energy you can extract by putting two nuclei together gets
05:06smaller and smaller the bigger the nuclei are until you come to making iron.
05:13And iron, it turns out, is the last thing you can make that way.
05:16The problem with iron is when you fuse it, it doesn't make energy, it takes it away.
05:23So when the star builds up that iron core, it's doomed.
05:27It can no longer create energy in its core to flow out toward the surface strong enough
05:33to keep it from collapsing.
05:34So collapse is what they do.
05:39In a fraction of a second, the star's core collapses down from the size of a planet to
05:45about the size of a small city.
05:49And when that happens, all hell breaks loose.
05:56A huge amount of energy is suddenly released, which forces the collapsing layers back out.
06:03The result?
06:04An enormous explosion we call a supernova.
06:13The shockwave from a supernova rips out at thousands of miles per second, and for a brief
06:18period of time, they're brighter than an entire galaxy.
06:23A supernova could devastate life on Earth, and the evidence can be found at the bottom
06:32of our oceans.
06:33There are layers and layers of silt that have built up, and there seemed to be a layer about
06:382.6 million years ago that was enriched in a very strange chemical element, something
06:43called iron-60.
06:45Iron-60 is a radioactive isotope of iron, and it doesn't last very long, just a few
06:50million years.
06:51And the only place that we know of that can make iron-60 is a supernova in an exploding
06:57star.
07:05That means there must have been a supernova close enough to the Earth within the past
07:10couple of million years to have physically deposited material on our planet.
07:18That freaks me out.
07:21The sign of this shocking assault on our planet is a thin layer of this very rare type of
07:26iron.
07:27We find it in the mud of every ocean floor, and always at the same depth.
07:35This interstellar dust must have drenched our world in one enormous burst 2.6 million
07:41years ago.
07:42It was a terrible time.
07:44A third of large animal species in the sea suddenly died out.
07:50There were some pretty amazing fish.
07:51Probably the most amazing is the megalodon, the giant shark, teeth the size of dinner
07:56plates and so on.
07:57But they went extinct 2.6 million years ago at the end of the Pliocene.
08:02What happened?
08:06A lot of sea creatures died, and a lot of them were in shallow waters, whereas deep
08:11water animals tended to survive.
08:14Well, that sounds kind of like a supernova.
08:16That can do things that would affect our atmosphere, would affect shallow water, but not deeper
08:21water.
08:24Supernovas create huge amounts of cosmic rays.
08:29When they crash into other atoms, they break up and produce showers of dangerous shrapnel
08:35called muons.
08:38These charged particles are similar to electrons, only 200 times heavier.
08:45So they penetrate more deeply and cause more damage.
08:50They can pierce through our atmosphere, pierce through our skin, get into a cell and disrupt
08:55the DNA.
08:56They'll go right through a mouse, but deposit in the body of a larger animal.
09:01So the impact on an animal the size of a megalodon, say, could be pretty extreme.
09:08Muons can shatter DNA, causing mutations and cancer.
09:12But their power weakens as they travel through water, which may be why only deep sea creatures
09:19survived.
09:21The extinction really tells us that we're not separate and apart from the universe and
09:26the goings on up there, right?
09:28Supernova going off and things like that, okay, it's a pretty light show.
09:30No, there's a direct impact to life on Earth and us.
09:36So are we in danger of extinction?
09:40Is Betelgeuse about to explode?
09:50When stars explode as supernovas, they can devastate planets hundreds of light years
09:56away.
09:58Betelgeuse is about 550 light years from Earth.
10:02So when it dramatically dimmed in 2019, scientists were concerned.
10:10But Betelgeuse has dimmed before.
10:15Betelgeuse varies quite a lot over the years.
10:18There are some cycles, and sometimes these cycles come together and you get a deep minimum.
10:24So dimming is part of the star's natural cycle as it nears the end of its life.
10:32But to get a full picture, we took Betelgeuse's temperature.
10:37If the star was dimming, that would mean that the surface was cooling over time.
10:41We actually made measurements of the temperature of Betelgeuse and found out that wasn't happening.
10:45It hardly cooled at all.
10:46It cooled like 50 or 100 degrees.
10:49You might expect a much, much more dramatic change in the surface temperature if it were
10:53about to explode.
10:57So if Betelgeuse wasn't cooling much, what was making it dim?
11:04To take a closer look, we used the Very Large Telescope and an exoplanet hunting instrument
11:12called SPHERE and came up with an extraordinary image.
11:19When I first saw this image of Betelgeuse, it blew me away.
11:23I almost gasped.
11:24I may have said a word I can't say on TV.
11:28That was very exciting.
11:31The image reveals that while the upper part of Betelgeuse was still bright, the lower
11:37part was noticeably dimmer.
11:40We had images of Betelgeuse from before, and we were able to compare the new ones with
11:45it.
11:46And so you could see that half of Betelgeuse looked pretty much the same, but the other
11:50half was significantly dimmer.
11:53And what could make a star dim that quickly?
11:56And remember how big this star is.
11:58Nothing happens on Betelgeuse quickly.
12:01So this must be something happening right on the surface.
12:06As heavier material, like silicon, emerges from the surface of Betelgeuse, it cools and
12:13condenses.
12:14It's kind of like sticking the hose in the wrong end of your vacuum cleaner.
12:18Instead of pulling stuff in, it blows all this dust out into space.
12:25Betelgeuse has cosmic indigestion and is belching dust, which makes the star seem dim.
12:32But it's not over.
12:35All through 2020, Betelgeuse first brightened and then dimmed again.
12:42So astronomers are watching this massive star with bated breath.
12:48It's going to explode.
12:49The question is, when?
12:51It's probably sometime in the next 100,000 years, but it could be tomorrow, it could
12:55have already exploded, and we're just waiting to see the light.
12:59With luck, if Betelgeuse blows, all we'll see is a beautiful light show.
13:06At a distance of 550 light-years, it's probably too far to do serious damage.
13:15But is there another star we should worry about?
13:22A closer star, just 150 light-years from Earth, could do us some major damage.
13:29A star like IK Pegasi.
13:33But it isn't this star, which we can see in our night sky, that's the threat.
13:40The main star is only about 1.6 times the mass of the sun.
13:43That's nowhere near enough mass to go supernova.
13:46And yet, we think it is the progenitor for a supernova.
13:50How can that be?
13:52The main star isn't alone.
13:56It has a more dangerous accomplice.
14:01There's another star there, orbiting the larger star.
14:05And this is what we call a binary system, two stars orbiting each other.
14:09Right now the system is stable, but things aren't always going to be the way they are
14:13now.
14:14And sometime in the future, things are going to change a lot.
14:20IK Pegasi is really made up of IK Pegasi A, a large white star, and its accomplice, a
14:30white dwarf called IK Pegasi B. This tiny star is the real threat to Earth.
14:41You can think of a white dwarf as a zombie.
14:45You know, it's a dead star, and they can eat living stars.
14:49If there's a normal star like the sun near a white dwarf, the white dwarf has very, very
14:55intense gravity.
14:56It can literally pull material off that normal star, and that material will then pile up
15:01on the surface of the white dwarf.
15:03So it really is eating a living star.
15:08These stars orbit each other just 18.5 million miles apart.
15:14That's closer than Mercury is to our sun.
15:17But they're not interacting with each other, yet.
15:22The problem is, sometime in the future, that normal star is going to run out of fuel.
15:27And when it does, it's going to expand into a red giant.
15:32When it gets to the end of its life, IK Pegasi A will cool and swell up to become a red giant.
15:41And that's it.
15:42No big explosion.
15:44It won't become a supernova.
15:48But that's just when its accomplice, IK Pegasi B, will start to feed.
15:56A lot of that material will gravitationally be attracted to the white dwarf and fall onto
16:00the surface.
16:02As the white dwarf pulls material from its bloated red giant neighbor, it gets more and
16:07more massive.
16:09Its gravitational pull increases, so it feeds even faster.
16:16The thing about a white dwarf is that on its surface, the gravity can be 100,000 times
16:22the gravity of Earth.
16:23So it's very intense.
16:25The material from the star is mostly hydrogen.
16:28And this can pile up on the surface.
16:30And it's getting squeezed very hard.
16:32But that's how you fuse hydrogen.
16:34That's how you make a hydrogen bomb.
16:37It reaches an incredible temperature, 36 million degrees.
16:41It can actually become hot enough to ignite nuclear fusion reactions.
16:45But right on the surface of the white dwarf, you get this bright flash, a nova.
16:53A nova is a big explosion, but not big enough to destroy the white dwarf.
17:00So once things calm down, the white dwarf starts feeding again until there's another
17:05nova.
17:06And another.
17:08And another.
17:10Each time, the white dwarf gets heavier.
17:19Eventually, it can no longer support its own weight.
17:24The core of the star is actually very dense.
17:26In fact, if you had like a teaspoon of material, it would weigh about as much as an 18-wheel
17:30truck.
17:31And it's basically right at the limit of normal matter being able to hold up at that density.
17:36But you dump more and more stuff onto it, and eventually there's a limit that's reached.
17:41And either collapses or, more generally, blows up.
17:51When this happens, IK Pegasi will be brighter than the full moon in our sky.
17:58Because it's only 150 light-years away.
18:01Having a supernova 150 light-years sounds like a bad idea, and it is.
18:06That's close enough that it might have some physical effects on the Earth.
18:13Right now, IK Pegasi is about as far from Earth as the supernova suspected of killing
18:19off the megalodon.
18:23So, how worried should we be?
18:26The good news is, the IK Peg system is moving away from the Sun and the Earth right now
18:31at a decent clip.
18:33So if it's not going to blow up for a while, that means it could be on the other side of
18:37the galaxy by the time it does.
18:40By that time, we'll be completely safe.
18:43In our hunt for the next supernova, our leading suspects are massive stars.
18:49And they don't come much bigger than P Cygni.
18:53It's about 30 times the mass of our Sun.
18:57Double the mass of Betelgeuse.
19:00So big, we don't call it a giant or even a supergiant.
19:05It's a hypergiant.
19:08And it's behaving suspiciously.
19:12It was first spotted back in 1600.
19:16But just a couple decades after that, it disappeared.
19:19And then reappeared again.
19:21And then disappeared again.
19:22And then like a hundred years later, reappeared.
19:27P Cygni is a luminous, blue variable.
19:30The rarest kind of stars in the solar system.
19:34It's a supergiant.
19:36It's a hypergiant.
19:39And it's behaving suspiciously.
19:42In the universe.
19:44This star is so unusual.
19:46We think because the core is producing an enormous amount of energy
19:51that's right on the tipping point of being able to rip apart the star itself.
19:59Stellar adolescents like P Cygni are erratic.
20:03They burn fiercely and they die young.
20:07They have some of the shortest lives of any star.
20:10Just a few million years.
20:12Which is why we see so few of them.
20:16It looks like P Cygni is going to go supernova any day now.
20:21And for all we know, it could blow up tonight.
20:30At a distance of about 5,000 light years,
20:34P Cygni is too far to threaten Earth.
20:37But we could learn a lot from it
20:40if only we could spot any signs that it's about to blow up.
20:46As an astronomer, and an astronomer who has studied supernovas professionally,
20:51having them far away is fine with me.
20:53Close enough that we can study them well,
20:55but not so close that I can study them personally on a physical level,
20:59on my own body.
21:00Yeah, no.
21:01A close supernova would be devastating for life on Earth.
21:07Will there be any warning signs
21:11before one of our prime suspects
21:14is about to blow?
21:31To find a supernova warning signal,
21:34we need to know what's happening
21:36deep inside the core of an exploding star.
21:41At the very beginning of a supernova explosion,
21:44the core of a massive star is collapsing.
21:46There's no more nuclear fusion going on,
21:48and it is compressing to higher and higher densities.
21:52The star's gravity crushes protons and electrons so close together
21:57they merge to form neutrons.
22:01The star's core becomes one of the densest materials in the universe.
22:07It's like a gigantic atomic nucleus.
22:10Roughly half a million Earths
22:13compressed into the volume, the size of a city.
22:16That's really, really dense stuff.
22:19If you had about a teaspoonful of material,
22:22that would be about as much mass as Mount Everest.
22:25Forcing protons and electrons together
22:28releases a huge amount of energy
22:31in the form of tiny, elusive,
22:34subatomic particles called neutrinos.
22:38Neutrinos are one of the most abundant particles in the universe,
22:42but they don't interact with things very much at all.
22:46Neutrinos are often called ghost particles
22:49because they do what ghosts do.
22:51They walk through walls.
22:53But neutrinos walk through us,
22:55they walk through the planet,
22:57they walk through stars.
22:59They're super ghosts.
23:02At first, these neutrinos can fly
23:05straight out of the core of the star.
23:08But as the star collapses,
23:10it gets so dense that some neutrinos get trapped
23:13and their energy turned into heat.
23:17And that creates a shockwave that rips the star apart,
23:20and the ensuing explosion
23:22is brighter than billions of stars all put together.
23:28This light show may be spectacular,
23:31but it's only 1% of the energy released in a supernova.
23:35The rest is in the form of a massive burst of neutrinos.
23:39So neutrinos could act as a supernova early warning system.
23:44At least, that's the idea.
23:47On February 24, 1987,
23:50that idea was tested.
23:55An astronomer was doing a routine survey
23:58of a dwarf galaxy close to ours.
24:02He was taking pictures of it,
24:04develops the pictures,
24:06and says, hey, there's a star here
24:08that wasn't there, you know, yesterday.
24:13He basically got up, walked outside,
24:16and looked and went, oh, there's that star.
24:19And it turns out he had discovered a supernova.
24:24Because it was the first supernova spotted that year,
24:27it was called Supernova 1987A.
24:321987A was an amazing event in the world of astronomy.
24:36Essentially, a supernova went off in our own backyard.
24:41It was very close to us,
24:43it was very close to us,
24:45occurring in a neighbor galaxy of the Milky Way.
24:48And so it was the brightest thing seen in our skies
24:52since the invention of the telescope.
24:57Supernova 1987A blazed with the power of a hundred million suns.
25:04But that wasn't the most exciting part.
25:07For the first time, we received an early warning
25:10that a supernova was about to appear
25:12three hours before it lit up our night sky.
25:17Neutrino observatories around the world
25:20saw a sudden surge in neutrinos
25:22from the same direction on the sky.
25:31Neutrinos' ability to zip across the galaxy,
25:34slipping through stars and planets like ghosts,
25:37gives them an unbeatable head start during a supernova.
25:42The neutrinos are released
25:44in the very earliest moments of the supernova blast.
25:48And they slip through the atmosphere of the star
25:51before it goes boom.
25:56Neutrinos can escape in as little as 10 seconds.
26:01But it can take hours for the shockwave
26:03to travel right through the star
26:05and blast off the outer layers, revealing the light.
26:09The result is that we see neutrinos
26:11from a supernova explosion
26:13before we see the actual light.
26:20So if we want to spot the next supernova explosion,
26:23we've got to be paying attention to the neutrinos.
26:30Astronomers set up the Supernova Early Warning System.
26:35A network of neutrino detectors all around the world.
26:41It should give astronomers
26:43several hours' notice of an impending supernova.
26:49But so far, nothing.
26:51No supernovas have occurred
26:53near enough for the system to detect.
26:57Neutrinos are like the friend that never comes.
27:00We're sitting here waiting for them,
27:02but we don't know when it's going to actually happen.
27:05Our hunt for the Milky Way's next supernova
27:09has identified some potential suspects.
27:14Very massive, lonely stars.
27:17And stars with smaller sidekicks.
27:22In 2018, astronomers found a system
27:25called APEP, 8,000 light-years away,
27:29with two very massive stars.
27:32Each one about as massive as Betelgeuse.
27:37These are giant stars,
27:40nearing the end of their lives
27:42with massive outer layers of gas
27:46that continually contract and heat up again and again.
27:51They become really huge and bloated and swollen.
27:55And they're prone to huge outbursts.
27:58These unstable stars
28:01are called Wolf-Rayet stars.
28:07They're very rare.
28:09And so hot and bright,
28:11they emit more radiation
28:13than a million sun-like stars.
28:16This intense energy
28:18is blasting their outer layers off into space.
28:22Mass loss has been occurring from the star
28:25so much so that you've actually lost
28:27all the hydrogen that wasn't burned into helium.
28:31So now you have a star
28:33that's made entirely of helium and heavier elements.
28:37With no hydrogen left,
28:39these massive stars are running low on usable fuel.
28:44They're like ticking time bombs,
28:46made even more dangerous
28:48because they're spinning so fast.
28:52It's spinning so quickly,
28:54it's on the verge of ripping itself apart.
28:57And this means that when this thing blows,
28:59it's gonna blow hard.
29:02When a star goes supernova,
29:04its core collapses.
29:06The smaller it gets,
29:08the faster it spins.
29:10Some cores collapse
29:12into fast-spinning neutron stars.
29:15Heavier ones, like APEP,
29:17collapse into even denser
29:19and more mysterious objects.
29:23Black holes.
29:27The immense gravity within APEP's collapsing core
29:30will drag back some of the gas and dust
29:33into a spinning disk.
29:37As the material falls onto the core,
29:40it compresses and it speeds up.
29:44The dying star spins faster and faster
29:47as it collapses.
29:50In this incredible rotation,
29:52it drives the creation
29:54of massive magnetic fields
29:56that are capable of funneling
29:58material around and up and out
30:01in the form of huge beams of radiation.
30:07So the energy from the supernova collapse,
30:10instead of being emitted spherically
30:12in every direction,
30:13comes at us in a tightly focused beam.
30:17Like a laser from the death star,
30:20it is pointed in one direction.
30:25This is a gamma-ray burst.
30:29It is the single scariest thing
30:31the universe has to offer.
30:33This is an explosion so powerful
30:36that in a few seconds or minutes,
30:39it can release as much energy
30:41as the sun will over its entire lifetime.
30:45You do not want to get caught
30:48in a gamma-ray burst.
30:49Let's just put it that way.
30:56The impact of a nearby gamma-ray burst
30:58on our home planet
31:00is almost too terrible to think about.
31:03It would be a very bad day for Earth.
31:09Earth's atmosphere
31:10could be partly blown away
31:12and there could be chemical reactions
31:14in the atmosphere
31:15that would form all kinds
31:16of noxious products.
31:19A gamma-ray burst from APET
31:22might last only 10 seconds,
31:24but its impact would last for decades.
31:27The generation of nitrogen oxide
31:29from a gamma-ray burst
31:31would be disastrous.
31:33In the upper atmosphere,
31:34it would eat away at our ozone layer.
31:36In the lower atmosphere,
31:38it would come out as acid rain.
31:41And the acid rain
31:42would destroy our crops.
31:46Nitrogen dioxide
31:48also filters out sunlight,
31:50turning the skies dark
31:52and cooling the Earth
31:53enough to trigger
31:54a new ice age.
31:59Any life on the land
32:01in the shallow parts of the sea
32:03or that live near the sea surface
32:05would be done.
32:06In fact, it would
32:07ultimately result in extinction.
32:14Blasted by ultraviolet radiation
32:16from our sun,
32:17freezing cold and hungry,
32:20humanity's future would be bleak.
32:26So we really need to know.
32:28When APEP goes supernova
32:30and produces its deadly beam
32:32of gamma rays,
32:35are we in its line of fire?
32:39The good news is that
32:40we are probably not right
32:42in the direct firing line
32:43of APEP.
32:44The axis of rotation
32:46of the APEP system
32:47is pointed 30 degrees
32:49away from us.
32:50So if it does blow,
32:52it's likely that the jets
32:54are going to miss us.
32:56Makes me feel better
32:57that this gamma ray burst
32:59isn't pointing at us.
33:00But of course,
33:01there are many other
33:02cosmic catastrophes
33:03potentially waiting to get us.
33:06APEP is on the edge
33:08of an enormous explosion.
33:10Its huge gravity
33:11and incredible spin
33:13should produce
33:14a spectacular supernova.
33:19But what if some stars
33:22are too big to blow?
33:35Galaxy NGC 6946,
33:39a local galaxy
33:40just 20 million light years away
33:43and well known
33:44to supernova detectives.
33:47It's the fireworks galaxy
33:48because it has produced
33:49so many supernovas
33:50in the past century.
33:52And they noticed
33:53that one star
33:54that they thought
33:55would become a supernova
33:56instead blinked out.
34:00The star under investigation
34:02is N6946BH1,
34:06a cosmic heavyweight
34:0825 times the mass
34:10of our sun.
34:11That's way more
34:12than the eight solar masses
34:14we thought
34:15guaranteed a supernova.
34:18This is a very massive,
34:19very luminous star.
34:21The prototype
34:22of what you expect
34:23to explode as a supernova.
34:26And over the last
34:27couple of years,
34:28its brightness has been changing.
34:30Maybe the star
34:31was beginning to go
34:32a bit unstable.
34:33But then,
34:34right in front of our eyes,
34:35this star
34:36just completely disappeared.
34:38This is a huge mystery.
34:40Why didn't this thing
34:41blow up?
34:43How can a star
34:44just disappear?
34:47There had to be
34:48something left behind.
34:51So astronomers
34:52began a search
34:53for evidence
34:54and found a crucial clue.
34:58When you look
34:59in the infrared,
35:00you could still
35:01see some light there.
35:02So there was a light
35:03in the infrared.
35:04But there was no light
35:05in the infrared.
35:06You could still
35:07see some light there.
35:08So there was
35:09something happening there.
35:10But what?
35:12We think
35:13the infrared light
35:14is heat
35:15coming off the debris
35:16of the dead star.
35:20Something
35:21is pulling it inwards.
35:23Something powerful,
35:24but also invisible.
35:27A black hole.
35:30The outer stuff
35:31from the star
35:32is still falling
35:33onto that black hole
35:34and it's powering
35:35a little bit of light.
35:36A little bit of the
35:37infrared light
35:38still gets out.
35:42How can a giant star
35:43become a black hole
35:44without exploding
35:45into a supernova first?
35:50The answer
35:51lies in how
35:52dying stars
35:53burn their fuel.
35:56For stars that are
35:57about, say,
35:5820 times the mass
35:59of the sun,
36:00you're actually
36:01going to burn
36:02things convectively.
36:03That means the gases
36:04in the atmosphere
36:05are moving around.
36:06A good analogy
36:07is water
36:08in a boiling pot
36:09of water.
36:10You've got
36:11your potatoes up here.
36:12You're trying
36:13to boil them.
36:14You've got
36:15convective cells
36:16of water
36:17that are heated.
36:18Bring the heat
36:19up to the top,
36:20get the potatoes hot,
36:21and then those
36:22blobs of water
36:23cool down,
36:24become denser,
36:25and settle down
36:26to the bottom again
36:27where they're
36:28heated once more.
36:30As fusion turns
36:31hydrogen to helium
36:32and then to carbon,
36:33convection
36:34mixes the carbon,
36:35so it burns up.
36:39Convection cells
36:41work inside
36:42of a star
36:43like massive
36:44elevators
36:45that take
36:46hot gas
36:47from the central
36:48regions,
36:49bring it up
36:50to the surface,
36:51allow it to cool,
36:52and then pull
36:53that material
36:54back down.
36:55They're constantly
36:56churning
36:57back and forth
36:58inside of a star.
37:01But stars
37:02more massive
37:03than roughly
37:0420 times
37:05the mass
37:06of the sun,
37:07like N6946BH1,
37:08don't burn
37:09carbon this way.
37:13Instead of mixing,
37:14the heavier atoms
37:15created by the
37:16fusion reactions
37:17just start
37:18to pile up.
37:21That means
37:22there's a layer
37:23of very dense
37:24material building
37:25up on just
37:26the surface
37:27of the core.
37:28All of the stuff
37:29is just ready
37:30to collapse.
37:32It's so powerful
37:33that if you have
37:34enough mass
37:35sitting around,
37:36the collapse
37:37is so powerful
37:38that it actually
37:39collapses into
37:40a black hole
37:41before any
37:42supernova goes off.
37:43That then
37:44is a failed supernova.
37:45It's a star
37:46that pretty much
37:47directly collapses
37:48to form a black hole.
37:52If many of
37:53the massive stars
37:54we expect
37:55to go supernova
37:56won't,
37:57that's a problem.
37:58We used to think
37:59we had the basics
38:00of supernovas cracked.
38:01Anytime you have
38:02a star more massive
38:03than eight times
38:04the mass of the sun,
38:05it was destined
38:06to explode
38:07as a supernova.
38:08And then along
38:09comes a star
38:10that screws everything up.
38:13To make things worse,
38:14we found
38:15no clear distinction
38:16between stars
38:17that go out
38:18with a bang
38:19and those that don't.
38:23As many as
38:2430 percent
38:25of massive stars
38:26could die
38:27without exploding.
38:29Our search
38:30for the next
38:31killer supernova
38:32is getting even harder.
38:34Stars blow up
38:35when we don't
38:36expect them to.
38:37They don't blow up
38:38when we expect them to.
38:39They can have
38:40several stars
38:41orbiting each other
38:42and the one
38:43that blows up
38:44isn't necessarily
38:45the one you think it will.
38:47So right now,
38:48we can't identify
38:49a prime suspect.
38:51But the hunt continues.
38:54As far as we know,
38:55there are no
38:56life-threatening
38:57stars out there,
38:59but we haven't
39:00done a complete survey.
39:02So please keep
39:03funding astronomy
39:04so we can
39:05keep looking.
39:10Supernovas
39:11are spectacular,
39:14devastating,
39:16and frightening.
39:20But,
39:21without them,
39:22we wouldn't exist.
39:23The iron
39:24in your blood
39:25and the calcium
39:26in your bones
39:27was literally
39:28forged inside
39:29of a star
39:30that exploded
39:31billions of years ago
39:32as a supernova.
39:33And I think
39:34this is one of the
39:35most beautiful
39:36and the most profound
39:37things that we've
39:38learned in astronomy,
39:39that we're
39:40literally,
39:41viscerally
39:42connected to the cosmos
39:43and the cosmos
39:44is connected to us.
39:47With every breath,
39:49we are inhaling
39:50oxygen
39:51that was created
39:52in a supernova
39:53explosion.
39:57This is almost
39:58literally
39:59a cosmic
40:00cycle of life.
40:07And a supernova
40:08may even be
40:09responsible for
40:10the dawning
40:11of our intelligence
40:12by causing
40:14lightning.
40:16It might sound
40:17rather incredible,
40:18but a supernova
40:19might actually
40:20influence directly
40:21the cosmic rays
40:22from a supernova
40:23will create
40:24charges
40:25in the lower
40:26atmosphere.
40:27That energy
40:28will break apart
40:29molecules,
40:30excite atoms
40:31and molecules,
40:32and it will
40:33ionize them.
40:34And an ionized
40:35atmosphere
40:36means that
40:37now
40:38it can conduct
40:39electricity.
40:40So it probably
40:41increased lightning
40:42across the planet.
40:45It's possible
40:46the same
40:47gamma ray burst
40:48that caused
40:49a mass extinction
40:502.6 million
40:51years ago
40:53also affected
40:54Earth's atmosphere
40:57triggering tremendous
40:58bursts of lightning
41:00which caused
41:02forest fires.
41:05We have evidence
41:06of widespread
41:07fires at this time.
41:09So it could be
41:10that lightning
41:11was increased
41:12and that created
41:13more fires.
41:14And those fires
41:15could have leveled
41:16forests and savannas
41:17creating grasslands.
41:18So how could
41:19this change
41:20boost our
41:21intelligence?
41:23With their forest
41:24homes burnt
41:25our ancestors
41:26early hominids
41:27had to adapt
41:28to life out
41:29in the open
41:30which meant
41:31standing up.
41:33You're living in a savanna
41:34where there's lions
41:35and leopards
41:36and cheetahs
41:37and the savanna
41:38is mostly grassland
41:39it's a lot more
41:40efficient
41:41perhaps
41:42on two feet.
41:43You can run
41:44and moving on two feet
41:45might have been
41:46the survival mechanism.
41:48Standing upright
41:49also triggered
41:50the most important
41:51change in our history.
41:54Walking around
41:55on two feet
41:56freed our hands
41:57to be able
41:58to start doing things
41:59and as you
42:00you know of course
42:01you can imagine
42:02as you start doing things
42:03it drives your brain
42:04to more complexity
42:05as you're trying
42:06to figure out
42:07how to manipulate things.
42:08And this is perhaps
42:09the biggest
42:10evolutionary leap
42:11because without it
42:12we don't get tool use
42:13we don't get fire
42:14we don't get intelligence.
42:16As our
42:17ancient ancestors
42:18adapted to
42:19their new habitat
42:20they took their
42:21first steps
42:22toward world domination.
42:25At least
42:26that's the theory.
42:28The idea
42:29presented here
42:30is
42:31this would be
42:32the dawn
42:33of modern humans
42:34as we see it
42:35and we would
42:36owe that
42:37to lightning
42:38created from
42:39a gamma ray burst.
42:40That's nuts.
42:43Supernovas
42:44are
42:45extraordinary.
42:47They launched
42:48our journey
42:49into the cosmos
42:50and in time
42:51a supernova
42:52may end it.
42:55We're
42:56searching hard
42:57to spot
42:58which one
42:59it could be
43:00but
43:01for now
43:02the only way
43:03we'll know for sure
43:04is when it
43:05lights up our sky.
43:08While a supernova
43:09might appear
43:10to be
43:11the death
43:12of a star
43:13the beauty
43:14is that it's
43:15really a story
43:16about beginnings
43:17as well.
43:23Supernovae
43:24giveth
43:25and they
43:26taketh away.
43:27Without supernovae
43:28the earth
43:29wouldn't exist
43:30and we
43:31wouldn't exist.
43:32I actually
43:33do imagine
43:34standing out
43:35on a nice
43:36winter night
43:37looking up
43:38at Betelgeuse
43:39and actually
43:40seeing the thing
43:41explode.
43:42There would
43:43be a light
43:44show.
43:46I would love
43:47to see a supernova
43:48up close, right?
43:49I mean,
43:50what a light show
43:51but there's
43:52no way
43:53I would want to
43:54be that close
43:55because I don't
43:56want to die.